Object.getOwnPropertyNames vs Object.keys

There is a little difference. Object.getOwnPropertyNames(a) returns all own properties of the object a. Object.keys(a) returns all enumerable own properties. It means that if you define your object properties without making some of them enumerable: false these two methods will give you the same result.

It's easy to test:

var a = {};
Object.defineProperties(a, {
    one: {enumerable: true, value: 1},
    two: {enumerable: false, value: 2},
});
Object.keys(a); // ["one"]
Object.getOwnPropertyNames(a); // ["one", "two"]

If you define a property without providing property attributes descriptor (meaning you don't use Object.defineProperties), for example:

a.test = 21;

then such property becomes an enumerable automatically and both methods produce the same array.

Another difference is in case of array Object.getOwnPropertyNames method will return an extra property that is length.

var x = ["a", "b", "c", "d"];
Object.keys(x);  //[ '0', '1', '2', '3' ]
Object.getOwnPropertyNames(x);  //[ '0', '1', '2', '3', 'length' ]